These computer designed and CNC laser cut cold-rolled steel brackets are designed to mount on your existing 5 on 5" drum spindles with no machine work or modification required.  They use 1976 Riviera rotors and 1971-76 Cadillac DeVille calipers.  They may fit inside the stock Buick 15" rims.  You will, however, need to realign the toe-in setting.